42 Proceedings of Royal Society of Edinburgh. [sess. 
distant points in succession during the determination of the 
hysteresis loop, the “ step-by-step ” method being in this case the 
only one available. To secure approximate symmetry about the 
zero of induction, the maximum cyclic value is first reached by 
reversals of field increasing from zero, the oscillations being “on.” 
After twenty reversals the oscillations are put off. The field is 
now decreased by the first step and the oscillations put on and 
off. The second step is next taken, followed by oscillations on 
and off as before. Three galvanometer readings are taken at each 
step ; the first reading measures the change of induction due to 
change of field, the second that due to oscillations, and the third 
any small change if any that may take place when the oscillations 
are put off. By proceeding in this manner the whole cycle may 
be completed. 
The dotted line curve is the hysteresis loop under the B con- 
ditions, i.e. when the field cycle is superposed upon permanently - 
acting oscillations. The full line curve is the normal hysteresis 
loop {i.e. without oscillations) determined by the step-by-step 
method for the same maximum values of field. 
